Abstract: Taking a fault in a Mertiari-Lahore DC converter out of step as an example. By taking emergency disposal measures on site, further deviations in the shifting gear were avoided and the occurrence of an accident with converter saturation protection or zero sequence current protection was prevented. Through on-site shifting mechanisms, AC power checks and secondary circuit measurements, the direct cause of the accident was eventually determined to be a fault in the AC power monitoring relay, which led to an unsuccessful AC power switch, resulting in the loss of AC power to the converter shifting mechanism. Countermeasures for the accident were proposed to quickly locate the fault and shorten the fault disposal time.